The invention is related to communication systems in general, and more particularly to cellular communications systems having means for determining the location of a mobile station.
A cellular system consists of an FM radio network covering a set of geographical areas (known as Cells) inside of which mobile two-way radio units, like Cellular Telephones, can communicate. The radio network is defined by a set of base stations distributed over the area of system coverage, managed and controlled by a centralized or decentralized digital switch equipment known as MTSO, or Mobile Telephone Switching Office. A base station in its geographical placement is known as a cell site. It is composed of low powered FM transceivers, power amplifiers, control unit, and other hardware depending on the system configuration. Its function is to interface between cellular mobiles and the MTSO. It communicates with the MTSO over dedicated data links, wire or non-wire, and communicates with mobiles over the air waves. The MTSO""s function is controlling call processing, call setup, and release which includes signaling, supervision, switching and allocating RF channels. MTSO also provides a centralized administration and maintenance point for the entire network. It interfaces with Public Switched Telephone Network (PSTN), over wire line voice facility, to honor services to and from conventional wire line telephones. At present, there is no way to locate wireless callers automatically when they seek emergency assistance.
When an emergency phone call initiates from a wireline telephone (using the PSTN), local exchange carrier switching software routes the call. This software associates the calling telephone number (ANI) with address information (ALI) stored in a location database and routes the call to-a centralized Public Safety Answering Point (PSAP).
When a PSAP receives an emergency call from a wireless location (using cellular, PCS, or Specialized Mobile Radio (ESMR) technology), neither ANI nor ALI information is available to the call taker. Agents must rely on the caller""s ability to provide location information. Without ANI, the 911 call taker cannot re-contract the emergency caller to obtain additional information either.
Issued in June 1996, Docket 94-102 creates rules to govern the availability of basic 911 services and the implementation of enhanced 911 (E 911) for wireless services.
Phase 1 requires wireless carriers to transmit 911 emergency calls to a PSAP identifying both calling mobile unit (ANI) and cell site/cell site sector (pANI). The emergency caller must transmit a Mobile Identification Number (MIN) or its equivalent, and the local 911 district must request ANI transmission from the wireless carriers. Phase 1 compliance is required by 1998.
Phase 2 requires wireless carriers to relay an emergency caller""s number, allow PSAP attendants to redial the caller in case of disconnection, and relay the location of the base station or cell site receiving the 911 call and its phase information. By 2001, the location of the mobile station must by provided to the PSAP in two dimensions (x,y) accurate within a radius of 125 meters in 67% of all cases.
Accordingly, it is highly desirable to obtain a method of locating wireless callers when they seek emergency assistance without significantly modifying the existing cellular network software and topology.
A system for determining the location of a mobile station in a cellular communications network, comprising a plurality of locator units fixedly positioned for tracking and measuring communications between a base station and a mobile station initiating an emergency communication, each locator unit comprising a receiving means for monitoring a control channel of the base station for detecting and receiving identifying information including an assigned voice channel associated with the particular mobile station initiating said emergency communication, a storage means for storing the identifying information associated with the emergency communication, a controller means for tuning the receiving means to the assigned voice channel associated with the emergency communication for receiving voice and control channel data between the base station and the mobile station, a transmission means operable in a first mode responsive to the receiving means and the controller means for initiating a second communication to a monitor unit based on the stored identifying information, the second communication occurring further including data indicative of the signal strength as a function of distance of the mobile station relative to the particular locator unit, wherein the monitor unit is responsive to the plurality of locator units for receiving the second communication from each locator unit and determining and tracking the position of the mobile station based on the received signal strengths and the known positions of the fixed locator units.